1. Field of the Invention
The present invention relates to a technique to judge a cause of an abnormality generated in a structure having a plurality of motors acting with each other.
2. Description of Related Art
Japanese Laid-Open Patent Application No. 2003-166135 suggests a technique to detect an abnormality in a load applied to a motor. In this abnormality detection technique, it is judged that an abnormality occurs in a load applied to a motor when a detected current flowing through the motor exceeds a threshold value. This patent document also discloses a technique to detect an existence of an abnormality from changes in a value of the current flowing through for the purpose of detecting an abnormality in a load being applied to the motor.
The technology in which an existence of abnormality in a load is determined based on only whether a current value exceeds a threshold value is effective in a case where the load is driven independently. However, for example, in a case where a primary transfer belt and a secondary transfer roller of an image forming apparatus that are driven by separate drive sources, it may be difficult to make an accurate determination because interference between the belt and the roller may give influences to the load conditions of both the belt and the roller. For example, if a primary transfer belt and a surface of a secondary roller are rotationally moved at the same speed and if the secondary roller expands due to a temperature rise, the primary transfer belt may be moved together with the secondary transfer roller because the moving speed of the surface of the secondary transfer belt is increased even when the drive motors for driving the primary transfer belt and the secondary transfer roller are controlled to maintain the same speed. In such a case, the load applied to the drive motor for driving the secondary transfer roller is increased by an amount corresponding to a force to move the secondary transfer roller. On the other hand, the drive motor for driving the primary transfer belt is decreased by an amount corresponding to the force given by the secondary transfer roller. Hereinafter, such a phenomenon that the secondary transfer roller is partially moved by the first transfer belt is referred to as “move together”.
Because of the influence of interference mentioned above, if, for example, an abnormality occurs that the load of the primary transfer belt is decreased, it is difficult to accurately determine whether the abnormality is caused by only a change in the load of the primary transfer belt or the expansion of the secondary transfer roller. If an accurate determination cannot be made, an appropriate control cannot be performed when such an abnormality occurs.